The Wyoming Department of Education (WDE) has invested time and money developing standardized achievement test score reports designed to give teachers data about each of their students' levels of mastery of particular concepts in order to differentiate their instruction. The purpose of this study was to determine the extent to which eighth-grade mathematics teachers used Traffic Signal Project Reports to analyze how teachers used data to differentiate instruction. A qualitative case study methodology was used in gathering data from teachers of similar backgrounds and school settings. This study focused on three research questions including how teachers used Traffic Signal Project Reports to inform their instruction, how teachers differentiated their instruction based on assessment data, and how teachers decided which sources of data they used to make instructional changes in the classroom. A panel of experts, seventh- and eighth-grade Wyoming math teachers, analyzed a set of interview questions and determined that they were adequate for the purpose of the case study. Questions focused on whether teachers used Traffic Signal Project Reports and/or other types of data to shape their instruction. The population of this study was middle school mathematics teachers located in Wyoming. Ten eighth-grade mathematics teachers from Title I schools and schools with similar demographics participated in the study. Results from the study showed that standardized test score data were used mostly for grouping students in classes by ability rather than used for differentiating instruction within the classroom. Also, teachers were more comfortable using their own classroom data pieces to differentiate instruction in the classroom rather than standardized achievement test score data. Furthermore, teachers utilized various forms of data to make instructional decisions such as class groupings or adjusting curriculum. The majority of the teachers either implied or stressed the need to utilize multiple sources of data before making instructional decisions. The teachers also indicated that the Traffic Signal Project Reports should be modified to make them more user-friendly for differentiation. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]